The invention relates to a piezoelectric actuator, for example for actuating a mechanical component such as a valve or the like, according to the features contained in the preamble to the main claim.
It is generally known that using the so-called piezoelectric effect, a piezoelectric element can be constructed out of a material with a suitable crystalline structure. The application of an external electrical voltage causes a mechanical reaction of the piezoelectric element which, depending on the crystalline structure and the region to which the electrical voltage is applied, produces a compression or tension in a predictable direction. This piezoelectric actuator can be constructed of a number of layers (multilayered actuators), wherein the electrodes by means of which the electrical voltage is applied are respectively disposed between the layers.
Piezoelectric actuators of this kind can be provided, for example, to drive on-off valves in fuel injection systems in motor vehicles. During operation of the piezoelectric actuator here, particular care must be taken that mechanical stresses in the layer structure do not also cause undesirable fractures to form in the vicinity of the external connecting electrodes. Since the internal electrodes, which are each contacted on one side, are integrated into the layer structure in comb fashion, the electrodes disposed one after another in the direction of the layer structure must be respectively contacted in an alternating fashion on opposite sides from one another. When the piezoelectric actuator is actuated, i.e. when a voltage is applied between the internal electrodes on opposite sides in the layer structure, different mechanical forces are produced in the vicinity of the internal electrodes as well as in the vicinity of the contacts on the external electrodes, which can lead to mechanical stresses and therefore to fractures in the external electrodes.
The piezoelectric actuator described at the beginning, which can be used, for example, to actuate a mechanical component, is advantageously modified by virtue of the fact that at least one layer of each respective external electrode is embodied as distributed in a net-like or woven cloth-like fashion on a respective side surface and is contacted at least at points by the respective internal electrodes. Between the contacts here, there is a respective expandable region and the electrical voltage is supplied by means of connections which likewise have expandable regions in the vicinity of the contact surface or between the contact points.
In a particularly preferred embodiment of the invention, as a first layer of the external electrode, a metal surface, for example, is disposed on a respective side surface which is contacted by the respective internal electrodes. As a second external electrode, the net-like or woven cloth-like layer is then placed upon the first layer, wherein it is contacted by the first layer at least at points and wherein an expandable region is disposed between the contacts in this instance as well. Here, too, the electrical voltage is then supplied by means of connections to the second external electrode, which likewise has expandable regions in the vicinity of the contact surface or between the contact points.
The net-like or woven cloth-like external electrodes are used to prevent lateral fractures which can arise from the inner electrodes due to delamination, as mentioned at the beginning. The net-like or woven cloth-like external electrodes can thereby be advantageously soldered in place only at points so that no large shearing stresses occur in the external electrodes during the expansion of the piezoelectric actuator, which can cause the electrodes to fracture.
In a modified embodiment, the connections for the electrical voltage can be produced by means of a solder contact which is disposed in a region of the layer structure of the piezoelectric actuator which does not have any active longitudinal expansion because the internal electrodes do not extend to the edge of the piezoelectric actuator.
In another embodiment, the connections for the electrical voltage can be advantageously produced by means of a soldered, welded, or crimped contact which is disposed in a folded region of the net-like or woven cloth-like external electrode. In this case, it is possible that the connections are produced by means of a flat wire which is disposed inside the folded region.
It is also possible that the connections for the electrical voltage are produced by means of a wire which is disposed linearly in the folded region that extends the entire length of the piezoelectric actuator or also by means of a wire that is laterally or perpendicularly wavy and a soldered, welded, or crimped contact, which is disposed at points on the woven cloth-like external electrode.
Furthermore, the connections for the electrical voltage can also be respectively produced by means of a comb electrode and a soldered or welded contact which is disposed at points on the preferably elastic combs and on the second external electrode.
These and other features of preferred modifications of the invention ensue not only from the claims but also from the specification and the drawings, wherein the individual features can be respectively realized singly or multiply in the form of sub-combinations in the embodiment of the invention and in other areas and can represent advantageous and intrinsically patentable embodiments which are claimed herein.